1. Field of the Invention
The present invention relates to a pneumatic booster attached to a brake apparatus for a vehicle such as an automobile.
2. Description of the Related Art
Typically, a pneumatic booster is attached to a brake apparatus for an automobile so as to generate a large braking force. Known examples, of a pneumatic booster include a pneumatic booster in which a housing is divided, by means of a power piston, into a constant pressure chamber (which is maintained at negative pressure at all times by a negative intake pressure of an engine) and a variable pressure chamber. A valve body is connected to the power piston, and a plunger provided inside the valve body is moved by means of an input rod to introduce atmospheric air (positive pressure) into the variable pressure chamber, thus generating a pressure differential between the constant pressure chamber and the variable pressure chamber. A thrust force generated in the power piston by the pressure differential is applied to an output rod through a reaction member, and a reaction force of the output rod which acts oh the reaction member is partly transmitted back to the input rod.
In a pneumatic booster of this type, the relationship between an input force (an operating force acting on a brake pedal) and an output force (a braking force) is as indicated by the solid line in FIG. 8. As shown in FIG. 8, in an initial stage of a braking operation, a “jump-in” output A is generated due to the existence of a gap between the plunger and the reaction member. Thereafter, the output force increases linearly in proportion to the input force to reach a full-load point B.
Needless to say, in the conventional pneumatic booster described above, since the braking force and brake pedal operating force are linearly commensurate, a large operating force is required to generate a large braking force in an event of an emergency. Therefore, to reduce the brake pedal operating force when a large braking force is required in the event of an emergency, a pneumatic booster comprising a so-called brake assisting mechanism is desirable. A pneumatic booster comprising a brake assisting mechanism, in combination with an anti-lock brake apparatus which prevents wheel lock during braking, can be expected to remarkably improve the braking force in an event of an emergency.
A pneumatic booster comprising a brake assisting mechanism is disclosed in Japanese Unexamined Patent Application Publication No. 2000-25603, for example. In this type of pneumatic booster, a plunger is associated with the expansion and contraction of a spring so that when the brake pedal depression force exceeds a fixed value in an event of an emergency, the spring is compressed, and the plunger is displaced greatly relative to the valve body. Thus, a boosting ratio increases rapidly as indicated by a portion C in FIG. 8, enabling a large braking force to be generated.